River restoration

Wednesday

Dec 14, 2016 at 2:01 AM

At least two river restoration projects on Cape Cod demonstrate the value of repairing wetlands, which benefits the local economy, increases coastal resiliency from rising seas and storm surges, restores habitat for wildlife, fish and shellfish and filters pollution.

Projects like the Coonamessett River restoration in Falmouth and the Herring River restoration in Wellfleet and Truro are long overdue. While the two projects are different in scope, they have much in common.

The $3.5 million project to restore the lower Coonamessett River will begin next year when a dam and sandy topsoil from decades of cranberry farming are removed to bring back the area's natural ecosystem.

The $50 million Herring River restoration will remove a 1909 dike and restore natural tidal flow to one of the largest wetlands in New England. Removing the dike in stages will make large areas of town a lot wetter. Roads will need to be raised, some holes of a golf course reconfigured, and numerous private homeowners satisfied.

But the benefits of restoration far outweigh the risks. Consider this from the final environmental impact statement of the Herring River project: "Because tidal restrictions radically affect the process of sedimentation on the salt marsh, much of the diked Herring River flood plain has subsided up to 3 feet. Coastal marshes must increase in elevation at a rate equal to, or greater than, the rate of sea-level rise in order to persist. This increase in elevation (accretion) depends on several processes, including transport of sediment and its deposition onto the marsh surface during high tides. This sediment transport must occur to promote the growth of salt marsh vegetation and gradually increase the elevation of the marsh surface. Diking has effectively blocked sediment from reaching the Herring River flood plain."

In Falmouth, the work on town-owned land between Route 28 and John Parker Road will bring river herring and trout back to the river, said Betsy Gladfelter, a member of the town’s Conservation Commission.

One dam at the lower end of the rivershed will be removed in the first phase, with a second dam to be removed later, along with the replacement of a culvert under John Parker Road.

On Cape Cod, more than 38 percent of our salt marshes have been lost to damming, dredging, filling, ditching and other human-related activities, according to the Association to Preserve Cape Cod. "Dikes and undersize culverts have restricted the natural tidal flow of seawater, causing formerly healthy salt marshes to become clogged with invasive vegetation and to atrophy," wrote Ed DeWitt, executive director of the Association to Preserve Cape Cod, in an op-ed to the Times in 2014. "More than 7,000 acres of salt marsh have been destroyed. In terms of flood storage capacity that has been lost, that amounts to 2.2 billion gallons of seawater for every foot in elevation."

Salt marshes are critical for healthy and abundant fish and shellfish, DeWitt said. "It’s estimated that 75 percent of our commercially important fish and shellfish species depend on salt marsh habitat at some point in their life cycles, serving as nurseries and a source of food."

Salt marshes also mitigate the rise in greenhouse gases by storing more carbon per acre than tropical rain forests. And they are among the best natural water cleansing filters, because they take up and store pollutants.

Although there has been success with restoration projects, much more needs to be done, DeWitt said. The Cape Cod and the Buzzards Bay atlases list more than 200 salt marshes with tidal restrictions.

"We need to step up our restoration efforts now," he said. "Successes, such as recent restoration projects in Barnstable, Brewster, Eastham, Harwich, Sandwich and elsewhere, point out the value and cost effectiveness of salt marsh restoration."